Base Color RVT not working with a material using "Use Material Attributes"

Development Rendering
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Hello,
My issue should be simple, but I can’t find an answer anywhere. Simply my Base Color RVT is not working with a material using “Use Material Attributes”.
When my MM_Landscape is not using material attributes and in the “Layer Blend” I just have plugged in simple vector colors everything is fine, but when I flag the “Use Material Attributes” (placing GetMaterialAttribute and SetMaterialAttribute because I want to have a full set of textures for each layer), the base color RVT becomes grey and I don’t know why.
The Height RVT seems working fine in both cases.

Thanks in advance to whoever has a solution for it.

Best regards,